For your budget, take into account the money you save by cycling to work. I've only have my latest bike 2 years and have paid it off 4 times over already by savings in train fares. I suggest a flat bar road bike. It should have 25-28 mm slicks and no suspension which increases the weight. Giant CRX-3 comes to mind. Helmet - who cares, just get something that doesn't look too daggy.